Over the past decade, IC technology for use of an organic semicondutctor thin film transistor (organic TFT) has been proposed. The main attractive points of such circuitry come from the expectations for easy processing and adaptability to a flexible substrate. These advantages are expected to be utilized in the low cost IC technology to be appropriately applied to smart cards, electronic tags, displays and the like.
A general organic TFT is composed of a glass substrate, a gate electrode, a gate insulating film, a source electrode, a drain electrode and an organic semiconductor film. Variation of a voltage applied to the gate electrode (gate voltage: Vg) generates surplus or deficient charges at the interface between the gate insulating film and the organic semiconductor film so that the drain current (Id) flowing through the source electrode/organic semiconductor/drain electrode is varied to perform switching.
The physical quantities used for representing the performance of an organic TFT include mobility, on/off ratio and gate voltage threshold value. The mobility is proportional to the slope of the VId-Vg curve in the saturation region where Id (drain current) and Vg (gate voltage) are in a linear relationship, and represents the degree of easiness in current flow. The on/off ratio is represented by the strength ratio between the minimal Id and maximal Id as obtained by varying Vg. The gate voltage threshold is defined by the X intercept of the straight line tangent to the VId-Vg curve in the above-mentioned saturation region, and represents the gate voltage that causes switching.
The target values for the characteristics of an organic TFT are intended to attain those values for the a-Si TFT used in currently prevailing active matrix liquid crystal displays. More specifically, the target values include a mobility ranging from 0.3 to 1 cm2/Vs, an on/off ratio of 106 or higher, and a gate voltage threshold value ranging from 1 to 2 V.
Recent advances in TFT devices based on polymers are described, for example, in U.S. Pat. No. 5,596,208, U.S. Pat. No. 5,625,199 and U.S. Pat. No. 5,574,291. According to the descriptions of these patents, development of n-type and p-type polymer materials comes to permit easy actualization of complementary IC, as detailed particularly in U.S. Pat. No. 5,625,199.
Besides, several attempts have been made for improving the characteristics of the field effect transistor.
Among others, the carrier mobility that is an important parameter determining the performance of the field effect transistor has been attempted to be improved by utilization of organic semiconductor materials with a sufficiently extended π-conjugated system, improvement of the crystallinity of the thin films of organic semiconductor materials, improvement of the characteristics as p-type semiconductor through enhancing the donor property on the basis of introduction of methyl groups into the organic semiconductors materials, utilization of organic semiconductor materials produced from oligomers having a single degree of polymerization rather than organic semiconductor materials having a dispersion in the degree of polymerization, and the like.
According to the recent studies, it has been revealed that the characteristics of the organic TFT are correlated with the crystallinity of the organic semiconductor film irrespective as to whether the organic semiconductor is of the lower molecule type or of the higher molecule type. For example, a paper (A. R. Brown, D. M. de Leeuw, E. E. Havinga, and A. Pomp, “Synthetic Metals,” Vol. 68, pp. 65–70(1994)) has disclosed a finding that a high mobility and a high on/off ratio are not compatible with each other in an organic TFT that uses amorphous organic semiconductor film. Additionally, another paper (Y-Y. Lin, D. J. Gundlach, S. F. Nelson, and T. N. Jackson, “IEEE Transactions on Electron Devices,” Vol. 44, No. 8, pp. 1325–1331 (1997)) has disclosed a production method of an organic TFT that uses as the semiconductor layer therein a high crystallinity vapor-deposited film of pentacene, and a finding that the TFT has such high characteristics that the mobility is 0.62 cm2/Vs, the on/off ratio is 108 or more, and the gate threshold value is −18 V.
An attempt has been made to improve the crystallinity of the organic semiconductor film on the basis of an underlying layer that is provided beneath the organic semiconductor layer. Japanese Patent Application Laid-Open No. 7-206599 discloses a production method for oriented organic semiconductor films of oligothiophene compounds or the like by use of an oriented film of polytetrafluoroethylene (PTFE) as an underlying layer. In this case, the PTFE film is formed on the substrate surface by sliding a solid lump of PTFE under a certain pressure so that it is difficult to make the area of a substrate larger. Additionally, the molecules in the organic semiconductor layer are aligned uniformly along the orientation direction of the PTFE film so that the carrier conduction between the molecules becomes difficult to occur, and accordingly the expected characteristics are hardly obtained.
Additionally, Japanese Patent Application Laid-Open No. 9-232589 discloses a production method of an organic TFT that is provided with a film oriented so that the organic semiconductor layer is oriented along the direction connecting the source electrode and the drain electrode. In this case, the intermolecular conduction is also difficult to occur on the above-described grounds, and accordingly it is difficult to obtain the high characteristics.
Additionally, a paper (Y-Y. Lin, D. J. Gundlach, S. F. Nelson, and T. N. Jackson, “IEEE Electron Devices Letters,” Vol. 18, No. 12, pp. 606–608 (1997)) discloses a production of a high performance organic TFT by forming two deposited layers of pentacene on the surface of the gate insulating film coated with octadecyltrisilane that is a type of perpendicularly oriented film. In this case, the evaluation of the TFT characteristics uses the drain voltage of −80 V and the gate voltage of −100 V, which are too high as the voltages to be applied to a semiconductor element.
Additionally, Japanese Patent Laid-Open Application No. 2001-94107 discloses an organic semiconductor device in which a crystalline organic semiconductor layer is formed on a fluorine-based polymer layer of 0.3 to 10 nm in thickness formed by the dip method on the surface of the gate insulating layer. However, with such a scheme of production, an interface is formed between the gate insulating film and the fluorine-based polymer layer so that no high mobility can be expected, and the driving voltage should become large according to the described configuration. Furthermore, the crystalline layer in the organic semiconductor layer exhibits two peaks so that the orientation thereof is not satisfactory, and hence no large improvement in characteristics can be expected. Additionally, Japanese Patent Laid-Open Application No. 2001-94107 describes that the mobility is increased by the presence of the two different crystal axes which are borne by the pentacene. It is conceivable that this is an invention based on a mechanism essentially different from that of the present invention because according to the present invention the mobility of the transistor is increased with increasing C-axis orientation ratio.
For the purpose of improving the characteristics of the organic TFT, important are the improvement of the crystallinity of the organic semiconductor film, the improvement of the design for element constitution, and furthermore the provision of high performance to the insulating film, and the like.
The present invention has been made for the purpose of overcoming the above-described problems, and an object of the present invention is to provide an organic semiconductor element that can be made uniformly on a large area of a substrate, can largely modulate the drain current through the voltage applied to the gate electrode, and has a high mobility.
Furthermore, another object of the present invention is to provide an organic semiconductor element that is stable in operation, can be driven by a low voltage, is long in operating life, and can be produced by a simple and easy method.
Additionally, still another object of the present invention is to provide an active matrix type display device that utilizes the above-described organic semiconductor element or an organic semiconductor device that utilizes the organic semiconductor element as IC-card electronic tag.